This invention relates generally to a switching power supply and more particularly to dynamic load detection with primary-side sensing and feedback.
Conventional flyback power converters include a power stage for delivering electrical power from a power source to a load. A switch in the power stage electrically couples or decouples the load to the power source, and a switch controller coupled to the switch controls the on-time and off-time of the switch. The on-time and off-time of the switch may be modified by the controller based upon a feedback signal representing the output power, output voltage or output current to regulate the same. The energy is stored in the gap of a transformer when a switch is on and is transferred to the load when the switch is off. Regulation can be accomplished by, among other things, measuring the output current (or output voltage) and feeding that back to a primary side controller, which can be used to modify the on-time and off-time of the switch accordingly.
In order to improve cost performance and reduce over-all size, many commercially available isolated power supplies employ primary-only feedback and control. By sensing primary side signals during each ON and OFF cycle, the secondary output and load condition can be detected and thus be controlled and regulated. This includes both constant voltage and constant current modes of operation.
Many electronic devices require the power supply to provide a controlled and regulated power source over wide operating conditions, adding to the difficulty of primary-side sensing and control. Portable electronic devices such as smartphones and tablet computers are examples of such devices.
FIG. 1 illustrates an operating curve of an example switching power converter used to provide a controlled and regulated output to a load. Operating conditions presented to the switching power supply may occur while a load such as an electronic device is connected to the power supply or when a load is not connected. For example, in a Constant Voltage Mode (CVM) 101, the switching power supply supplies a regulated DC output of a fixed voltage within a certain tolerance range indicated by CVM range 104. CVM 101 generally indicates that the internal battery of the electronic device is fully charged and the fixed voltage output of the power supply provides the operating power for the electronic device to be operated normally.
In a Constant Current Mode (CCM) 102, the power supply provides a fixed current output. CCM 102 generally indicates that the internal battery of the electronic device is not fully charged and the constant current output of the power supply allows for the efficient charging of the internal battery of the electronic device.
Lastly, in a no-load condition 103, the electronic device is disconnected from the power supply. Under the no-load condition 103, the switching power supply may maintain a regulated voltage output within the CVM range 104 in anticipation of the electronic device being reconnected to the power supply.
For convenience, end users often leave the power supply connected to the AC mains at times where no load is connected to the power supply output. Because the power supply maintains a regulated output voltage even in no-load conditions, a dual-mode control methodology is commonly employed. During periods when there is a nominal load, pulse width modulation is employed. However, when the load approaches no load, it is difficult to maintain a duty-cycle low enough to maintain output regulation. Accordingly, a pre-load, or dummy load can be added, however, operational efficiency and no-load power consumption are negatively impacted. Furthermore, because power supplies are oftentimes connected to the AC-mains for long periods of time when they are not connected to the electronic device, government and environmental agencies have placed maximum limits on the no-load power consumption.
In such situations, one technique is for the controller to change its regulation mode under low load or no-load conditions. Under no-load conditions, the rate of the pulses that turn on or turn off the power switch of the switching power converter is decreased significantly in order to maintain output voltage regulation, resulting in long periods of time between ON and OFF cycles of the switching power converter. This presents a significant challenge to primary-side sensing control schemes that rely on the ON and OFF cycles of the power switch to obtain a feedback signal. During the periods between ON cycles of the switch, the status of the output voltage is unknown by the controller as no feedback signal is generated. If the electronic device is reconnected to the power supply, representing a dynamic load change, during one of the long OFF cycles of the switch, the primary-side controller does not receive feedback about the change in the secondary side output voltage until the next ON cycle of the switch. In the interim, the output voltage may therefore drop significantly, exceeding the allowable voltage drop specified by the regulation specifications.